Means for varying the picture brightness in an electron microscope



Aprxl 23, 1963 E. GUTTER 3,087,057

y MEANS FOR VARYING THE PICTURE BRIGHTNESS 1N AN ELEcTRoN MIcRoscoPE F so, 1959 iled Nov.

i f s. n

3,087,057 MEANS FOR VARYIPJG THE PICTURE BRIGHT- NESS IN AN ELECTRON MICRGSCOIE Ernst Gtter, Oberkoelien, Wurtternberg, Germany, as-

signor to Carl Zeiss, Heidenheim (Brenz), Wurttemberg,

Germany Filed Nov. 30, 1959, Ser. No. 856,218 Claims priority, application Germany Nov. 29, 1958 2 Claims. (Cl. Z50-49.5)

This invention relates to corpuscular ray apparatus and, more particularly, to corpuscular ray apparatus in which the image brightness is variable, and to an irnproved method of varying the image brightness.

In corpuscular ray apparatus, such as electron microscopes, it is frequently necessary to vary, within certain limits, the intensity of the ray or beam impinging on the object and thus the brightness of the image. For example, variation of the intensity of the beam is necessary when observing sensitive objects; variation or" the image brightness is necessary in order to be able to focus to equal image brightness despite extensive variations in magnification.

The variation of brightness of the image in corpuscular ray apparatus, particularly the electron microscopes, was heretofore elected by varying the intensity of the corpuscular ray or beam emitted from the source of radiation. The variation of beam intensity is effected by controllable adjustment of the potentials applied to one or more electrodes lying in the acceleration iield.

In the usual arrangement the controlled variation in electrode potential is eiected by an adjustable voltage superimposed upon the electrode bias potential. Thus, the adjustable voltage source rides at the high bias potential (about 40-100 kv. in the electron microscope). To protect the operator, the adjusting dial or other meinber provided for operator control of the amplitude of the adjustable voltage must be insulated from the voltage source itself. In addition to the expense of such construction, the user is always subject to the risk of a voltage -ashover at the insulator which, due to the high operating potentials, is always a possibility.

Another decisive disadvantage of the regulation of the image brightness by varying the intensity of the beam supplied by the beam generating system is that by a change in the potential of the control electrode, the optical conditions of the beam generating system are changed, resulting, for example, in the displacement of the position of the narrowest cross section of the beam from the object plane in which it should normally lie.

Further, changes of beam intensity cause concomitant changes in the operating voltages biasing the beam generating system. These changes are attributable to the high internal resistance of the high voltage sources biasing the beam generating system, which high internal resistance is mandatory for reasons of safety. Thus, whenever the intensity of the beam is changed, the entire electron-optic conditions in the apparatus change in an undesired manner,

It is therefore one object o f this invention to provide an improved method for controlling image brightness in corpuscular ray apparatus.

It is a further object of this invention to provide a corpuscular ray apparatus in which the beam is periodically interrupted in controllable fashion to vary the average energy impinging upon the image forming plate and thus to vary the image brightness.

It is a still further object of this invention to provide an improved corpuscular ray apparatus in which the beam intensity may be adjusted to the desired predeter.. mined intensity and the image brightness controlled by controlled interruption of the beam.

3,087,057 Patented Apr. 23, 1963 P t Q In accordance with the method of this invention, the corpuscular beam supplied by a beam generating system is adjusted to a iixed predetermined intensity. The beam is periodically interrupted with the time of interruption variable in accordance with operator control. Since the intensity of the beam is maintained constant, any change in the electron-optic conditions is avoided. The intermittent beam is used to produce the image with the average value determining the image brightness.

In a preferred embodiment of this invention the corpuscular ray apparatus is provided with a diaphragm dening the illumination aperture. Means are provided to cyclically deflect the beam across the aperture to provide an intermittent beam for image production.

In order to free the generating System from any disturbing eilects due to the beam deecting eld, it is advantageous to locate the beam deflection system in the iield free space in front of the object to be imaged.

In order to vary the brightness of the image, the amplitude of the beam deflection is varied. For instance, in the case of electrostatic beam deection by increasing the applied deflection voltage, the deflection -in the plane of the diaphragm becomes greater, whereby the time of passage across the diaphragm aperture is reduced with respect to the period and thus the average value in time, which alone can be observed, of the brightness of the image is reduced. In order to eliminate flickering of the image by the pulsating illumination, deflection frequencies of more than 1,000 cycles are advantageously employed.

The invention will be described in further detail below with reference to the embodiments shown in FIGURES 1 and 2 in which:

FIGURE 1 is a cross sectioned view of corpuscular ray apparatus in accordance with the present invention; and

FIGURE 2 is a cross sectioned view of a portion of another embodiment of a corpuscular ray apparatus in accordance with this invention.

In FIGURE 1 there is shown an electron beam generating system comprising a cathode 1, a control or modulator electrode 2, and an anode 3. The anode 3 is at ground potential, while all other electrodes are at high voltage. The electron beam 4, produced by the beam generating system, passes through a grounded diaphragm 5 which determines the maximum illumination aperture and impinges on the `object 6 which is to be observed. By means of an objective lens 7 another diaphragm 8 and a projection lens 9 an image of the object 6 is produced on a fluorescent screen 10. This fluorescent screen can be observed through a window 11.

Between the anode 3 and the diaphragm 5 there is arranged an electrostatic dellection system 12 which is connected with a source o f alternating voltage 13.

Before the observation, the electron'beam is adjusted to a predetermined intensity by a suitable selection of the bias voltage of the control electrode. This electron beam is periodically swept across the aperture in the diaphragm by the deection system 12. In this manner, an intermittent electron beam strikes the object 6'. The average value in time of the electron beam acting on the object 6 and thus also the brightness of the image produced on the lluorescent screen llt)l is determined by varying the amplitude of the alternating voltage fed to the de ection system 12,. If, for intance, the deflection voltage is reduced, then the passage time is increased as compared with the length of the period and the image becomes brighter. Similarly, by an increase of the deilection amplitude the brightness of the image is decreased.

Instead of the electrostatic deilection system 12 shown in FIGURE l, there can also be used, as shown in FIG- URE 2, an electromagnetic deilection System 14. The

there can be employed, in the simplest case, 'an electromagnetic deflection system consisting of two air coilsv located opposite each other which are connected in series and traversed by alternating current.

rIhis invention may be variously modified,and-Lem-v.v

bodied within the scope of the subjoined claims.

What is claimed is:

1. An electron microscope having a beam generating ing a beam defining aperture therein, said diaphragm positioned between the anode and the object to bef" system including an anode, comprising a diaphragm havl imaged, and means for cyclically sweeping the electron beam across said aperture in a symmetrical deflection pattern, said last named means comprising means for generating an alternating deection eld and means for varying the amplitude of said eld.

2. An apparatus in accordance with claim 1 in which said alternating eld frequency is greater than 1,000 cycles per second.

References Cited in the le of this patent UNITED STATES PATENTS 2,050,628 Prinz Aug. 11, 1936 2,217,197 Davisson Oct. 8, 1940 2,294,659 Herold Sept. 1, 1942 v7..,330,888 Hillier Oct. 5, 1943 v2,457,092 Simard Dec, 2l, 1948 v 2,602,751 Robinson July 8, 1952 '1 2,820,139 Adler Ian. 14, 1958 

1. AN ELECTRON MICROSCOPE HAVING A BEAM GENERATING SYSTEM INCLUDING AN ANODE, COMPRISING A DIAPHRAGM HAVING A BEAM DEFINING APERTURE THEREIN, SAID DIAPHRAGM POSITIONED BETWEEN THE ANODE AND THE OBJECT TO BE IMAGED, AND MEANS FOR CYCLICALLY SWEEPING THE ELECTRON BEAM ACROSS SAID APERTURE IN A SYMMETRICAL DEFLECTION PATTERN, SAID LAST NAMED MEANS COMPRISING MEANS FOR GENERATING AN ALTERNATING DEFLECTION FIELD AND MEANS FOR VARYING THE AMPLITUDE OF SAID FIELD. 